The present invention relates to an arrangement for the compensation of inertia forces in a machine element to which a reciprocating movement between an upper and a lower turning point is imparted.
It is very customary in mechanical engineering for a machine element to be driven in a repeated, reciprocating movement. In each direction of movement there occurs at least one acceleration and one deceleration of the machine element, and frequently an intermediate period also exists during which the machine element is driven at constant speed. If the machine member is large, that is to say, if it has a large mass, these constantly recurring changes of speed as well as of direction of movement require that very substantial driving forces must be applied with resulting oscillation and vibration effects. A special problem arises in the case where the path of movement of the machine element is more or less vertical, that is to say, when the machine element is to be displaced between an upper and a lower turning point and vice versa. In such cases the force of gravity will act upon the mass of the machine element so that the driving arrangement is always subjected to an additional stress in one direction.
A machine element with a pattern of motion as described above is described in Swedish patent application No. 7400444-1, to which reference is made. The machine element is driven by means of a motor-driven cam disc and a lever rod in a reciprocating motion between an upper and a lower turning point. The machine element has a large mass and is moved at non-uniform speed. More particularly the machine element is moved in a working stroke from the upper turning point to the lower one at constant speed (apart from a very short distance of acceleration and deceleration respectively in the upper and lower part of the working stroke). In other words, the element is accelerated from zero speed at the lower turning point during half the upward return stroke to the maximum speed. Subsequently, the element is immediately decelerated to zero speed at the upper turning point. Since the cycle time is relatively short (approx. one second) and the mass of the machine element is large (180 kg) a very substantial application of force is required for the driving which force produces oscillations and vibrations in the whole machine. With the intention of reducing these vibrations an arrangement in the form of a pneumatic piston and cylinder unit attached to the machine frame has been provided, whose piston rod is connected to the machine element and acts upon the same in upwards direction. The piston and cylinder unit is the single-acting type and the unit's working chamber is connected to an air tank of a volume which exceeds many times the volume of the working chamber. It is intended thereby to provide the piston with a substantially constant force directed upwards, irrespectively of the position of the piston in the cylinder. However, since the desired force to be applied varies during the total cycle time, the pressure in the working chamber of the piston and cylinder unit had to be calculated for the desired maximum force at a certain instant during the working cycle. During the remainder of the working cycle the piston and cylinder unit provides an excessively large force, which causes a varying stress on the driving unit. Therefore the solution described, in spite of providing a certain reduction in the vibrations, cannot be regarded as a completely satisfactory one.
It is an object of the present invention to provide an arrangement which by being specially adapted to the pattern of motion and mass of the machine element is capable of compensating the inertia forces arising so that an effective reduction of oscillations and shocks is achieved.
It is a further object of the present invention to provide an arrangement which by absorbing and storing energy makes it possible to reduce the amount of applied driving force required.
It is a further object of the present invention to provide an arrangement which is simple and uncomplicated and which, moreover, is not subject to the disadvantages of earlier designs.
These and other objects have been achieved in accordance with the invention by providing an arrangement for the compensation of inertia forces in a machine element to which can be imparted a reciprocating movement between an upper and a lower turning point, which arrangement comprises two spring elements. The first spring element balances the weight of the machine element and the second spring element tends to move the machine element in the direction from the lower turning point.
A preferred embodiment of the arrangement in accordance with the invention has been given the further characteristic that the second spring element tends to move the machine element to a substantially central position between the upper and the lower turning point.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the spring elements consist of first and second pneumatic piston and cylinder units which are arranged in series and connected to the movable machine element as well as to a fixed point.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the first piston and cylinder unit is of the single-acting type. The volume of the first unit's working chamber is enlarged by communication with an air tank whose volume is many times greater than the swept volume of the first piston and cylinder unit.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the pressure in the working chamber of the first piston and cylinder unit is substantially constant and so great that it balances the weight of the machine element.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the second piston and cylinder unit is the single-acting type and has a closed working chamber with a pressure at rest which slightly exceeds the pressure in the working chamber of the first piston and cylinder unit.
A further embodiment of the arrangement in accordance with the invention has been given the further characteristic that the length of stroke of the first spring element is smaller than the distance between the upper and lower turning point of the machine element.